The present invention relates to a semiconductor device for the transmission-reception of modulated optical signals on which are monolithically integrated a transmitter and an optical receiver, as well as means for reducing smearing between the transmitter and the receiver.
It is currently accepted that bidirectional communications will be provided by integrated optronic circuits having on the same circuit a transmitter e.g. transmitting on a wavelength of 1.3 xcexcm and a receiver receiving on a wavelength of 1.55 xcexcm or conversely transmitting on a wavelength of 1.55 xcexcm and receiving on a wavelength of 1.3 xcexcm. One point relative to the hopes and difficulties of implementing such circuits forms the object of an article in the xe2x80x9cJournal of Quantum Electronicsxe2x80x9d, according to reference [1] in the list of references attached to the present application.
One of the difficulties is due to smearing problems between the transmitter and receiver of the integrated circuit.
Although the transmission laser operates at a wavelength of e.g. 1.3 xcexcm and the reception photodiode is optimized in order to have a maximum sensitivity at a wavelength of 1.55 xcexcm, the photodiode is also sensitive to the wavelength of 1.3 xcexcm, transmitted in the vicinity by the laser with a power able to reach several hundred times the saturation level of the photodiode. This leads to the well known smearing problem.
Thus, European patent application EP 732 782 [2] describes an in-line integrated optronic circuit in which an absorbent of the wavelength transmitted by the laser is placed between the transmitter and the reception photodiode. This leads to a receiver with an improved signal-to-noise ratio.
Improvements to this manner of reducing smearing are described in European patent application EP 755 082 [3].
Several absorbing units are placed between the transmitter and the receiver. The increase in the number of absorbing units makes it possible to obtain an acceptable smearing of the order of xe2x88x9240 dB without any excessive increase in the length of the device. This solution is more efficient than that consisting of increasing the length of a single unit.
Patent application EP 795 910 [4] also describes an integrated optronic circuit having means for reducing between the transmitter and the receiver, in which the means for reducing smearing comprise an electro-absorbent placed between the transmitter and the receiver. The electro-absorbent has a forbidden band width between the forbidden band width of a laser material constituting an active layer of the transmitter and the forbidden band width of a laser material constituting an active layer of the receiver. These absorbing means can be completed by a supplementary electro-absorbing layer able to absorb the transmitter light.
The means described up to now are passive means with which the aim is to reduce the noise produced in the reception photodiode by the laser transmitter located in the vicinity thereof.
Attempts have also been made to reduce the noise caused by the transmitter in the receiver in an active manner. Thus, in an article [5] entitled xe2x80x9cFull Duplex Performance Assessment of In Line Transceivers Emitting at 1.3 xcexcm and Receiving at 1.55 xcexcm/sxe2x80x9d, the authors explain how they actively reduced smearing.
They used a structure having three units comprising a DFB transmission laser with a distributed reflector at 1.3 xcexcm, a Fabry Perot resonator laser replacing the reception photodiode at 1.55 xcexcm and an absorption unit at 1.3 xcexcm placed between the two laser units.
Experiments were carried out with the aim of evaluating the improvement which could be obtained by reducing the modulation coefficient mxe2x80x2 representing the modulation of the signal transmitted at 1.3 xcexcm as received at the receiver at 1.55 xcexcm.
Two methods were used.
In a first method, there is a simultaneous modulation of the signal transmitted by a data signal and the absorption unit by a signal complimentary of the data signal obtained by a phase shift of 180xc2x0 of the data signal.
According to the second method, the data signal is sent following attenuation and phase adjustment to an input of a differential amplifier receiving on a second input the signal detected by the reception photodiode at 1.55 xcexcm. Thus, the resulting signal at the output of the differential amplifier is a differential signal between the signal and the noise received by the photodiode and the data modulation signal of the transmitter at 1.3 xcexcm.
As a result of these procedures, it was possible to obtain a sensitivity of xe2x88x9223.9 dBm and xe2x88x9219.1 dBm for transmission rates of 68 Mb/S and 196 Mb/S respectively, in complete duplex communication.
These figures are close to those obtained when the laser at 1.3 xcexcm has not been modulated. As far as is known to the inventor the best results obtained on the publication date of document [5] with an integrated optronic circuit were obtained with an in-line transceiver circuit having an intermediate absorption unit. More recently with a new structure permitting the absorption of unguided light, for a modulated optical power of 2 mW peak-to-peak in transmission, a sensitivity above xe2x88x9231 dBm was measured for a complete duplex communication at 155 Mb. The disadvantage due to the complete duplex is evaluated as 1 dB. However, leaks of the transmission laser are always present and it has not been possible to obtain a similar sensitivity with a stronger modulation of the laser. At present, there is considered to be no way of avoiding extraneous leaks towards the reception photodiode due to the geometrical limitations resulting from the sought ever-reduced dimensions of the monolithically integrated optronic circuit.
The idea on which the invention is based is to actively reduce noise due to extraneous light of the transmitter by placing an extraneous light sensor in the vicinity of the detection photodiode. Thus, e.g. using a differential amplifier, the signal from the transducer is subtracted from the signal detected by the photodiode. In summarizing, the invention relates to a process for improving the detection sensitivity of a signal received by a receiver of an integrated optronic circuit, which is characterized in that the extraneous noise is detected in the vicinity of the receiver and from the signal received by the receiver is subtracted a compensation signal formed from the detected extraneous noise signal.
This process is implemented by a semiconductor device for the transmission-reception of modulated optical signals implemented on a semiconductor substrate, on which are monolithically integrated a first semiconductor component having a first active layer, said first component being a transmitter able to transmit a transmitted signal formed by a modulated light carried by a first wavelength, a second semiconductor component having a second active layer, said second component being a receiver able to detect a reception signal formed by a modulated light carried by a second wavelength, characterized in that said device also includes a sensor of a signal representing an extraneous noise and subtraction means for subtracting from the reception signal a compensation signal formed from the signal representing the extraneous noise detected by said sensor.
Thus, as a result of said device, by adjusting the phase and amplitude of the signal detected by the sensor of the monolithic circuit adjusted to the transmission wavelength, it is possible to compensate the leaks of the transmission signal and there is a significant improvement to the signal-to-noise ratio of the receiver.
The monolithic device can be an in-line device, as is e.g. described in patent application EP 755 082 A1 [3], or a surface component as is e.g. described in the article by METZGER et al published in the records of the 20th European conference on optical communications ECOC 94 held in Florence, Italy, pp 87-90 and entitled xe2x80x9cPhotonic integrated transceiver for access networkxe2x80x9d [6].
Preferably, the sensor of the extraneous signal and the receiver are similar semiconductor structures in order to have the same wavelength fluctuations with the temperature variations.